Abstract

Clastic sediments deposited in foreland basins and offshore fans allow the evolution of an orogen to be reconstructed—provided their source regions are properly characterized. Isotopic data from the lithotectonic units of the eastern Himalaya (Bhutan) indicate clear isotopic differences directly comparable with the equivalent units from the central Himalaya. Zircons from metaquartzites of the High Himalayan Series that range in age from 980 to 1820 Ma and an orthogneiss intruded at 825 ± 9 Ma bracket deposition to between 816 and 980 Ma. Model ages derived from Nd isotopes of associated metapelites range from 1700 to 2200 Ma. In contrast, zircons from a metaquartzite from the Lesser Himalayan Series range from 1850 to 2550 Ma. A Paleoproterozoic deposition age (ca. 1750 Ma) is inferred from the age of a metarhyolite associated with the sediments. Model Nd ages of metapelites from the Lesser Himalayan Series range from 2500 to 2600 Ma. The Bhutanese Himalayan units can be correlated more than 1000 km westward along strike from catchments of the Brahmaputra to the headwaters of the Sutlej on the basis of their structural position, provenance, and isotopes. Because current discharge from the Brahmaputra carries the dominant sedimentary flux into the Bengal Fan, results from this work validate the interpretation of isotopic variations observed in offshore deposits, in terms of the unroofing history of the component lithologies of the Himalaya.